1. Field of the Invention
The present invention generally relates to control of electron sources to achieve high uniformity and stability over time and, more particularly to control of a high-emittance source of a broad electron beam having substantially uniform electron flux over a large beam cross-section and suitable for use in an electron beam projection lithography tool.
2. Description of the Prior Art
At least one lithographic patterning process is invariably required in the manufacture of electronic integrated circuits. In view of the well-recognized benefits in functionality, performance and manufacturing economy which derive from increased levels of integration density, electron beams have been used to perform exposure of a layer of resist on a wafer at current and foreseeable minimum feature size regimes. However, exposure of entire chip patterns cannot be achieved at the present state of the art of electron beam lithography as has been possible with radiant energy exposure at larger minimum feature sizes and many serial exposures must generally be made for each chip.
To achieve economically acceptable levels of throughput using electron beam exposure, electron beam projection lithography has been used to allow exposure of a sub-field of the chip pattern which is formed in a reticle placed in the beam path (or to which the beam may be directed). By doing so, potentially several million pixels or image features can be simultaneously exposed reducing the number of serial exposures necessary for a chip by several orders of magnitude compared to exposure by a so-called probe forming electron beam exposure tool. Nevertheless, the area which can be covered by a sub-field is limited due to the high degree of resolution and freedom from aberrations which must be maintained to support very small feature size regimes and the complexity of the dynamics of the electron beam. For example, the charge carried by individual electrons causes mutual repulsion, referred to as Coulomb interactions, between them and the effects of these forces is proportional not only to the current in the beam but also electron density within discrete regions within a patterned beam.
To obtain the required pattern fidelity within the sub-field, the beam must be highly uniform over its cross-section prior to patterning. Further, for economically acceptable throughput, the beam current must be relatively high even though it is desirable to keep electron density in the beam as low as possible, consistent therewith. Accordingly, electron sources capable of such a function are often referred to as high-emittance, low brightness sources. Electron density is reduced by choosing the largest practical sub-field dimensions and a relatively large beam divergence angle.
The distribution of energies of the electrons in the beam is also relatively critical to calibration of the various electron-optical correctors (e.g. focus coils, stigmators and the like) in the electron beam tool. Relatively high beam energy is generally preferred to reduce the time over which Coulomb interactions can operate to disturb beam geometry.
A suitable electron source having the above properties has been proposed but is not admitted to be prior art in regard to the present invention. This electron source provides a broad, substantially uniform (or exhibiting a desired emission profile) electron beam using an indirectly heated cathode and provides for electron emission from a directly heated filament to impinge upon and cause further emission from the cathode. The surface of the cathode facing the filament may be contoured to adjust the bombardment electron flux incident on respective regions thereof to enhance emission uniformity. Edges of the beam are intercepted and thus truncated to limit the energy distribution of the electrons allowed to populate the beam.
However, while this electron source provides for good uniformity (or other desired emission profile) over a broad beam and acceptable limitation of electron energy spread, as with any electron source, emission efficiency of surfaces will vary with time and usage. Further, the relative electron emission fluxes from respective elements are interrelated in a complex manner (including radiative heating of the filament by the cathode and vice-versa in accordance with radically differing thermal masses). Long term loss of emission efficiency can, to some extent, be corrected by calibration but the useful lifetime of an electron source to provide acceptable image fidelity depends of the amount of correction available while producing acceptable uniformity of emission.
Another complication of regulation of cathode emission current in electron beam tools is the interrelationship of control parameters since electron emissions may occur which do not contribute to beam current. Further, more or less complex dependencies on chosen operating points and/or tool geometries may be presented. For example, even in a simple, two element, directly heated cathode arrangement, the proportion of electrons emitted which contribute to the beam will vary with accelerating voltage. These dependencies rapidly become much more complex when the emission source has increased numbers of elements (e.g. more than two) and operating currents/voltages respectively provided thereto.
Therefore, compensation of a change of performance of one element with a respective set point may affect the performance of another element and may not result in the desired correction to achieve constant emission current. Additionally, the temperature response of a filament is not instantaneous due to its thermal mass and is usually on the order of a significant fraction of a second which may be significant relative to the frequency of exposure at which an electron beam tool is operated. Moreover, the thermal mass of an indirectly heated cathode is many times larger than that of the filament and cathode response time is on the order of several seconds. Accordingly, unpredictable instabilities and hardware criticalities have complicated the problem of beam current regulation.
It is therefore an object of the present invention to provide a system for controlling a complex high-emittance electron source which stabilizes electron emission and electron energy over extended periods of time and operation.
It is another object of the invention to provide a control system for an electron source which effectively extends the useful lifetime of a high-emittance electron beam source.
It is a further object of the invention to provide a control system for an electron source capable of regulating electron emission and energy variation to negligibly low levels which maintains electron beam tool exposure uniformity.
It is yet another object of the invention to provide a control system for an electron source having only a single set point control.
In order to accomplish these and other objects of the invention, a system and method are provided for controlling voltage applied to a filament of an electron source using two or more nested feedback loops responsive to detected beam current, a set point, and current output of respective voltage source(s) providing acceleration of electrons.